Generally, the process of electrostatographic copying is initiated by exposing a light image of an original document to a substantially uniformly charged photoreceptive member. Exposing the charged photoreceptive member to a light image discharges the photoconductive surface thereof in areas corresponding to non-image areas in the original input document while maintaining the charge in image areas, resulting in the creation of an electrostatic latent image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by a process in which developing material is deposited onto the surface of the photoreceptive member. Typically, this developing material comprises carrier granules having toner particles adhering triboelectrically thereto, wherein the toner particles are electrostatically attracted from the carrier granules to the latent image for forming a powder toner image on the photoreceptive member. Alternatively, liquid developing materials comprising marking particles (or so-called toner solids) and charge directors dispersed in a carrier liquid have been utilized, wherein the liquid developing material is applied to the latent image with the marking particles being attracted toward the image areas to form a developed liquid image. Regardless of the type of developing material employed, the toner or marking particles of the developing material are attracted to the latent image and subsequently transferred from the photoreceptive member to a copy substrate, either directly or by way of an intermediate transfer member. Once on the copy substrate, the image may be permanently affixed to provide a "hard copy" reproduction of the original document or file. In a final step, the photoreceptive member is cleaned to remove any charge and/or residual developing material from the photoconductive surface in preparation for subsequent imaging cycles.
The above described electrostatographic reproduction process is well known and is useful for light lens copying from an original, as well as for printing applications involving electronically generated or stored originals. Analogous processes also exist in other printing applications such as, for example, digital laser printing where a latent image is formed on the photoconductive surface via a modulated laser beam, or ionographic printing and reproduction where charge is deposited on a charge retentive surface in response to electronically generated or stored images. Some of these printing processes develop toner on the discharged area, so-called DAD, or "write black" systems, while other printing processes, such as light lens generated image systems, develop toner on the charged areas, so-called CAD, or "write white" systems. The instant invention applies to systems which implement either of such printing processes.
The use of liquid developing materials in imaging processes is well known. Likewise, the art of developing electrostatographic latent images formed on a photoconductive surface with liquid developing materials is also well known. Indeed, liquid developing material-based systems have been shown to provide many advantages, and generally produce images of higher quality than images formed with dry toners. For example, images developed with liquid developing materials can be made to adhere to paper without a fixing or fusing step, thereby eliminating a requirement to include a resin in the liquid developing material for fusing purposes. In addition, the marking particles used in liquid developing material can be made to be very small without resulting in problems often associated with small particle powder toners, such as airborne contamination which can adversely affect machine reliability and can create potential health hazards. Development with liquid developing materials in full color imaging processes also tends to produce a texturally attractive output document due to minimal multilayer toner height build-up (whereas full color images developed with dry toners often exhibit substantial height build-up of the image in regions where color areas overlap). In addition, full color imaging with liquid developing materials is economically attractive, particularly if surplus liquid carrier containing the toner particles can be economically recovered without cross contamination of colorants. Further, full color prints made with liquid developing materials can be processed to a substantially uniform finish, whereas uniformity of finish is difficult to achieve with powder toners due to variations in the toner pile height as well as a need for thermal fusion, among other factors.
As previously indicated, liquid developing materials generally include a liquid phase, comprising an insulating carrier liquid such as an isoparaffinic hydrocarbon, and a solid phase, comprising marking particles composed of a pigment and a binder, as well as other optional materials, wherein the solid phase marking particles are dispersed or suspended in the liquid phase carrier. In addition, liquid developing materials further include a small amount of charge director for insuring that the marking particles are uniformly charged to the same polarity, either positive or negative depending upon the particular application. Charge director compounds are generally ionic compounds capable of imparting an electrical charge to marking particles of a desired polarity and a uniform magnitude so that the particles may be electrophoretically deposited on a charged surface (e.g., the photoreceptive member). The desired charging is achieved by providing a constant optimum concentration of charge director compound in the developing material liquid. If the liquid developing material contains excessive charge director, the developed images will tend to be somewhat faint due to loss of image charge caused by leakage in the higher conductivity liquid developing material. On the other hand, if the liquid developing material contains an insufficient amount of charge director, the developed images will also tend to be somewhat faint since marking particles having reduced charge move with reduced velocity through the liquid carrier to the imaging surface.
A more serious problem regarding liquid developing materials having insufficient charge director is that the marking particles tend to drop out of suspension, forming sludge deposits which continually grow until operation of the electrostatic copier must be interrupted for cleaning. In some liquid developing materials, it is the maintenance of the charge on the marking particles by the charge director which causes the particles to repel one another, maintaining them in a dispersed state, and preventing them from agglomerating and forming sludge deposits. Thus, stable electrical characteristics in the liquid developing material, in particular the bulk conductivity thereof, are critical in achieving high quality imaging, particularly in high speed, high volume applications. An important factor in determining the electrical characteristics of the liquid developing material, and affecting the electrophoretic development process, is the concentration of the charge director in the liquid carrier. Variation in the charge director concentration is a major problem in liquid developing material-based electrostatographic imaging systems.
In general, when a copy or print is made using liquid developing material, a constant amount of carrier liquid containing an associated amount of liquid phase charge director is deposited over the entire surface of the copy substrate. There is further deposited upon the copy substrate an amount of toner solids or marking particles proportional to the image areas being developed on the copy substrate. The marking particles also include an associated amount of solid phase, charge director and liquid carrier. Accordingly, during the development of a latent image, a first fixed amount of carrier liquid and charge director are depleted from the supply of liquid developing material, along with a second, variable quantity of liquid carrier and charge director associated with the marking particles. The depletion amounts of each of these components depends on the amount of image and non-image areas on the latent image being developed.
The present invention contemplates a liquid developing material control system wherein each individual component of the liquid developing material, namely the liquid carrier, the marking particles, and the charge director compound present in the liquid developing material are maintained at a predetermined optimal value in response to the amount of each component which is depleted from the supply of liquid developing material with each development cycle. Thus, in the present invention, a supply reservoir is maintained with a substantially constant amount of liquid carrier to which is added marking particles and charge director to provide an operative solution of liquid developing material. For each development cycle, the amount of liquid carrier, marking particles, and charge director depleted from the supply of liquid developing material is determined and added to the supply reservoir. More specifically, the amount of marking particles, liquid carrier and charge director depleted during a given development cycle is determined as a function of the number of picture elements or pixels making up the image being developed, for controlling the amount of marking particles, liquid carrier and charge director concentrate to be added to the supply reservoir to maintain an optimal operative solution of liquid developing material. The amount of liquid carrier depleted during a given development cycle is determined as a function of the number of picture elements making up the image being developed in combination with the rate of evaporation of the liquid carrier, for maintaining the amount of liquid carrier in the supply reservoir to maintain an optimal operative solution therein.
The following disclosures may be relevant to some aspects of the present invention: